Myotonic Dystrophy 1 (DM1) is a complex, inherited, neuro-muscular disease without cure. DM1 is caused by an expansion of untranslated CTG repeats within the 3'UTR of DMPK gene located on the chromosome 19q. We and others found that DM1 mutation causes a disease through accumulation of untranslated RNA CUG repeats which disrupt RNA processing in patients'cells. Mutant DMPK mRNA containing expanded CUG repeats is forming nuclear foci, disassembly of which is expected to reverse the disease phenotype. It is also expected that degradation of mutant DMPK mRNA would reduce toxicity of CUG repeats. However, latest data suggest that the short products of degradation of the mutant DMPK mRNA are more toxic than un-degraded RNA. (1) It has been recently found that the accumulation of multiple copies of short CUG repeats of normal size in mice produced DM1-like phenotype (myotonia, muscular dystrophy and cardiac conduction defects), while mice with low number of copies of long CUG repeats organized in nuclear foci were asymptomatic;(2) Our recent data on the mechanism of DM2, which is associated with expansion of untranslated CCTG repeats showed that accumulation of normal size of RNA CCUG repeats leads to DM2-specific abnormalities. Based on these data, we hypothesize that toxicity of CUG repeat expansion in DM1 patients is associated with the products of degradation of the mutant DMPK mRNA, while undegraded mutant RNA is not toxic. To examine this hypothesis, we propose to compare the effect of un-degraded (stable) and decayed CUG transcripts on DM1-specific pattern of RNA processing (translation and splicing). These studies are designed in Specific Aim 1. Time curve for mutant CUGn RNA decay will be determined after transcription pulse of CUGn-containing transcripts using tet-responsive promoter. We will examine whether nuclear CUG foci are formed by non-degraded or degraded mutant CUG repeats and whether CUG foci formation increases or reduces toxicity of CUGn repeats. We will determine the period of life of CUGn mutant transcript during which it has highest toxicity. The mechanisms regulating stability and decay of normal and mutant DMPK mRNA will be examined in Specific Aim 2. Regulatory regions within the 3'UTR of DMPK controlling DMPK mRNA turnover will be determined. Protein factors regulating normal and mutant DMPK mRNA stability and decay will be identified. The results of this study will identify status of mutant CUGn repeats when these transcripts become toxic and will help to develop approach to reduce CUGn RNA toxicity by controlling decay of the mutant DMPK mRNA. PUBLIC HEALTH RELEVANCE. DM1 is a devastating disease associated with toxicity of untranslated RNA containing repetitive CUG repeats. This project will identify status of CUG repeats when CUG repeats become toxic and will develop approach to reduce toxicity of these repeats at the level of CUGn RNA stability.